Surface acoustic wave filter and method for improved rejection

Abstract

A SAW coupled resonator filter providing improved rejection outside the passband frequencies includes at least two filter tracks on a piezoelectric substrate with each track having a plurality of acoustically cascaded transducers disposed along a longitudinal axis of the filter track. At least one filter track includes signal pads driven in a differential mode or a balanced mode. The two tracks are electrically connected with crossover bridges crossing over connecting lines between the tracks such that voltages across the crossover bridges are approximately the same in magnitude but substantially out-of-phase. The connecting lines extend between opposing bus bars of split transducers within each of the filter tracks.

1. A surface acoustic wave coupled resonator filter comprising:
 a piezoelectric substrate suitable for propagation of surface acoustic waves; 
 opposing filter tracks, each track having a plurality of acoustically cascaded transducers disposed upon the piezoelectric substrate along a longitudinal axis of the filter track, wherein the plurality of acoustically cascaded transducers includes a center transducer and external transducers on opposing sides thereof, and wherein the center transducer comprises a split transducer; 
 each of the filter tracks having a plurality of signal pads associated with the transducers and at least one of the filter track signal pads is driven in at least one of a differential mode and a balanced mode, wherein the opposing filter tracks are electrically connected by at least two connecting lines connected between the opposing split transducers; and 
 at least two crossover bridges, each crossover bridge crossing one of the at least two connecting lines, wherein voltages for the at least two crossover bridges are approximately the same, and wherein voltages for the at least two connecting lines are approximately the same in magnitude but are substantially out-of-phase. 
 
     
     
       2. The filter according to  claim 1 , wherein the external transducers are adjacent the center transducer. 
     
     
       3. The filter according to  claim 1 , wherein the split transducers each comprise a split bus bar, and wherein the split bus bar of the split transducer in one filter track is in an opposing position to the split bus bar of the split transducer in the opposing filter track. 
     
     
       4. The filter according to  claim 1 , wherein the plurality of transducers comprises three transducers. 
     
     
       5. The filter according to  claim 1 , further comprising reflector gratings carried within each filter track along the longitudinal axis and external the transducers. 
     
     
       6. The filter according to  claim 1 , wherein a signal input is obtained at the external transducers along a first filter track and driven in a single ended mode, and wherein a signal output is obtained at the external transducers of a second filter track and driven in the at least one of the differential mode and the balanced mode. 
     
     
       7. The filter according to  claim 6 , wherein at least one of the signal input and the signal output is provided by an electrical connection including the crossover bridges. 
     
     
       8. The filter according to  claim 1 , wherein at least one of an input signal and an output signal is provided by at least one of the signal pads through an electrical connection including the at least two crossover bridges. 
     
     
       9. The filter according to  claim 1 , wherein each of the split transducers comprises at least one split bus bar, and wherein the connecting lines transversely connect the at least one split bus bar of opposing transverse filter tracks. 
     
     
       10. The filter according to  claim 9 , wherein the bus bars of opposing external transducers provide signal pads for the signal input and the signal output. 
     
     
       11. The filter according to  claim 1 , wherein signal pads from the external transducers within one filter track provide an input signal and signal pads from the external transducers within the opposing filter track provide an output signal. 
     
     
       12. The filter according to  claim 1 , wherein at least one of the plurality of acoustically cascaded transducers comprises an unapodized transducer. 
     
     
       13. A surface acoustic wave filter comprising:
 a piezoelectric substrate; 
 at least two filter tracks, each track having a plurality of acoustically cascaded transducers disposed upon the piezoelectric substrate along a longitudinal axis thereof, wherein at least one transducer within each filter track comprises a split transducer, and wherein at least one of the filter tracks is driven in at least one of a differential mode and a balanced mode; 
 a plurality of connecting lines electrically connecting the at least two filter tracks by an electrical connection between the split transducers; and 
 a plurality of crossover bridges crossing over at least two of the plurality of connecting lines, wherein voltages for the plurality of crossover bridges are approximately the same, and wherein voltages for each pair of connecting lines of the plurality of connecting lines between corresponding split transducers are approximately the same in magnitude but are substantially out-of-phase. 
 
     
     
       14. The filter according to  claim 13 , wherein the plurality of acoustically cascaded transducers includes a center transducer and an external transducer on opposing sides thereof, and wherein the center transducer comprises the split transducer. 
     
     
       15. The filter according to  claim 14 , wherein the external transducers are adjacent the center transducer. 
     
     
       16. The filter according to  claim 13 , wherein the split transducers each comprise a split bus bar, and wherein the split bus bar of the split transducer in one filter track is in an opposing position to the split bus bar of the split transducer in the opposing filter track. 
     
     
       17. The filter according to  claim 13 , wherein the plurality of transducers comprises three transducers. 
     
     
       18. The filter according to  claim 13 , further comprising reflector gratings carried external to the transducers along the longitudinal axis. 
     
     
       19. The filter according to  claim 18 , wherein a signal input is obtained at the external transducer along a first filter track and driven in a single ended mode, and wherein a signal output is obtained at the external transducer of a second filter track and driven in the at least one of the differential mode and the balanced mode. 
     
     
       20. The filter according to  claim 13 , wherein at least one of an input signal and an output signal is provided by an electrical connection including the plurality of crossover bridges.